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This article is about the time interval used in astronomy. In astronomy, a Julian year is a unit of time defined as exactly 365.25 days or 31,557,600 seconds. The name derives from the fact that this corresponds to the average length of the year in the Julian calendar used in Western societies in previous centuries. However, it is fundamentally just a convenient intuitive way to measure large intervals of days (not "real" years such as tropical years or sidereal years), and there is no connection with year-month-day calendar timekeeping in the Julian or any other calendar. Julian years are used primarily for convenience in ephemeris work where stating a number of days would be unwieldy (for instance, it is easier to express the orbital period of Pluto as 248 Julian years rather than 90,590 days). It is an intuitively understood unit whose value is very close to the actual length of a year, yet intervals measured in Julian years can easily be converted to an interval in days without awkward long decimal fraction arithmetic. The "year" used in the definition of light year is a Julian year (note that a light year is a unit of distance, not time). Julian years are not to be confused with the Julian day (or Julian date), which is also used in astronomy. Despite the similarity of names, there is no connection between the two. A Julian year is not 365.25 "Julian days", it is simply 365.25 days. A Julian day (or Julian date) is not a unit of time, but simply a running count of days with an arbitrarily chosen starting point in the distant past, with each day numbering one greater than the previous; it's a way to specify a date without reference to months or years. On the other hand, a Julian year is a unit of time and not a running count of years. Astronomers would not say something like "this year is Julian year 2005" (which would be like saying "this hour is hour number 245"); rather they would use it in sentences like "the sidereal period of Pluto is 248.0208 Julian years".
Specifying and naming epochs Note that Julian years in astronomy are purely a unit for measuring time intervals and durations, and are not used as any kind of calendar or timekeeping system. Astronomers do not use the Julian calendar for modern events: when they need to mention a particular date (the date of a solar eclipse for instance), they use the Gregorian calendar like everyone else. However, for events before the introduction of the Gregorian calendar on 15 October 1582, the Julian calendar with a 1 January to 31 December historical year is used. Nevertheless, Julian years are the basis for naming standard epochs used in astronomy. An epoch simply specifies a precise moment in time. The Julian epoch J2000.0 is synchronized to exactly 12:00 TT (close to but not exactly Greenwich mean noon) on January 1 2000 in the Gregorian (not Julian!) calendar, and future epochs can be calculated and named according to the number of days since then, divided by 365.25. Thus the future epoch "J2100.0" will be exactly 36525 days from J2000. However, this is not really of any practical use as a calendar system (it is synchronized to the Gregorian calendar date for January 1 2000 and not the Julian calendar date with the same designation, which differ by nearly two weeks, yet it uses Julian years so it will diverge from the Gregorian calendar in a few hundred years). Rather, it is simply a convenient way to specify and name epochs. In astronomy, positions of stars (right ascension and declination, corresponding to geographic longitude and latitude) gradually change due to precession, so when specifying the coordinates of a star it is necessary to mention what epoch those coordinates apply to. Also the orbital elements of planets change slightly on a continuous basis due to the effects of gravitational perturbations, so when these are specified it is also necessary to specify what epoch applies. The standard epoch in use today is J2000, but for practical reasons a new standard epoch is chosen about every 50 years or so. Reference See also | ||||||||
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